Many thin image display devices as typified by displays, i.e., flat panel displays, have been released on the market, because they are thinner than cathode-ray tube displays and they do not occupy much space in depth. Their market price has decreased year by year with advances in production techniques, resulting in a further increase in demand and a yearly increase in production. Especially, color LCD TVs have almost become the mainstream of TVs. Also in recent years, light-emitting display devices including organic light-emitting display devices such as organic EL displays, which provide high visibility by light emitted by themselves, have received attention as the next generation of image display devices. In relation to the performance of these image display devices, there is a strong demand for a further increase in image quality, such as an increase in contrast and color reproducibility, and a decrease in power consumption.
A color filter is used in these liquid crystal display devices and light-emitting display devices. For example, in the case of color LCDs, the amount of light is controlled by using a backlight as the light source and electrically driving the liquid crystal. The light passes through the color filter and represents colors. Accordingly, the color filter is indispensable for color representation in LCD TVs and plays a large role in determining display performance. In organic light-emitting display devices, a color image is formed in the same manner as liquid crystal display devices, when the color filter is used in combination with an organic, white light-emitting element.
A recent trend is that there is a demand for power-saving image display devices. To increase backlight use efficiency, there is a very high demand for high-luminance color filters. This is a major issue especially for mobile displays such as mobile phones, smart phones and tablet PCs.
Even though technological advances have increased battery capacity, there is still a limit on battery capacity of mobile devices. Meanwhile, there is a trend that power consumption has grown with the increase in screen size. Image display devices including a color filter determine the design and performance of mobile terminal devices, because they are directly linked to the usable time and charging frequency of mobile terminal devices.
In general, a color filter includes a transparent substrate, color layers made of color patterns of the three primary colors (red, green and blue), and a light shielding part formed on the transparent substrate so as to define each color pattern.
To form such color layers, a pigment dispersion method in which pigments with excellent heat resistance and light resistance are used as color materials, has been widely used. However, it is difficult for color filters produced by use of pigments to satisfy the latest demand for higher luminance.
As a means to achieve higher luminance, dye-containing color resin compositions for a color filter have been studied. In general, dyes have a higher transmittance and can produce a higher-luminance color filter than pigments. However, dyes have a problem in that they are inferior in heat resistance and light resistance to pigments and the chromaticity is likely to change when they are heated at high temperature in a color filter production process, for example. Also, color resin compositions containing dyes have a problem in that cured coating films of the resin compositions are poor in solvent resistance, and the dyes transfer to adjacent pixels in other colors or to cured films that do not contain colorants (e.g., protective films). In addition, color resin compositions produced by use of dissolved dyes have many problems when they are used for color filter application. For example, aggregates are likely to precipitate on the surface of a cured coating film during a drying process, and a remarkable decrease in contrast is caused by the fluorescence of the dyes.
As a method for improving various kinds of resistance properties of dyes, a method for producing a salt-forming dye is known.
In Patent Document 1, a color photosensitive composition containing a dyed lake pigment is disclosed as a color photosensitive composition with excellent heat resistance, light resistance, color characteristics and transparency. Also, as the examples of the dyed lake pigment, publicly-known pigments with color index numbers are described.
However, in the dyed lake pigment disclosed in Patent Document 1, the ratio of the dye is small. Therefore, the dyed lake pigment is insufficient in coloring power, and a color layer using such a dyed lake pigment is insufficient in heat resistance and light resistance.
In Patent Document 2, an acidic dye aluminum lake pigment crystal is disclosed as a pigment with high coloring power and excellent light resistance, which is aimed at increasing the dye ratio in the pigment. However, due to large growth of the crystal particle diameter of the acidic dye aluminum lake pigment crystal of Patent Document 2, the pigment is poor in dispersibility, and it is difficult to obtain high contrast required of a color filter.
In Patent Document 3, a blue color composition for a color filter, the composition containing a colorant consisting of a copper phthalocyanine blue pigment and a metal lake pigment of a xanthene-based dye, is disclosed as a blue color composition for a color filter, the composition being capable of forming a color filter with high luminosity and a wide color reproduction range. In Patent Document 4, a blue color composition for a color filter, the composition containing a blue pigment and a salt-forming compound obtained by reaction of a xanthene-based acid dye and a compound containing a cationic group, is disclosed as a blue color composition for a color filter, the composition being capable of forming a color filter with high luminosity and a wide color reproduction range. However, an organic compound containing a quaternary ammonium salt group is merely disclosed as the compound containing the cationic group. The salt-forming compound obtained by use of the organic compound containing the quaternary ammonium salt group, has the following problems: a cured coating film of the composition is poor in solvent resistance; a sublimate is produced; and a decrease in contrast is caused by fluorescence.
In Patent Document 5, a color filter and so on are disclosed by the applicant of the present application, which use specific color materials containing divalent or higher anions and divalent or higher cations, in which dye skeletons are crosslinked by crosslinking groups. It is disclosed that the color materials are excellent in heat resistance since, due to containing the divalent or higher anions and the divalent or higher cations, molecular associations are formed therein, and color filters using the color materials are high in contrast and excellent in solvent resistance and electric reliability.